You only need to buy DLC if you're planning on cancelling ESO+ at some point in the future and you want to keep access to those areas after that.
ESO+ members have access to all DLC (except the most recent chapter - currently Elsweyr - which is a seperate purchase) but only as long as their subscription is active. If you cancel ESO+ you lose all those areas (but you won't lose any progress you've made, you just can't go back there) so you have the option to buy them and have permanent access if you want it.
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"
They have sales that offer the DLCs at more reasonable prices in case you thinking about dropping ESO+. Make sure you check what's in them. Some are questing, others are dungeons-focussed.